Apparatus of the kind mentioned above are already known to a certain extent, see EP-A No. 2 0186 854, (U.S. Pat. No. 4,625,180) for example. This known frequency synthesis apparatus operates intermittently for saving power, as with the present apparatus. In the known apparatus, there is a phase locked loop which is switched in or out. Three switches have been arranged to avoid frequency instability in the synthesizer, this instability being caused by phase uncertainty between the reference frequency and the output frequency of the voltage controlled oscillator. One of the switches is a power switch for switching in and out a supply source to the frequency divider for intermittent operation. The second and third switches switch in and out the reference generator and the frequency divider to the phase comparator in the loop, respectively.
This known frequency synthesizer operates intermittently but gives a continuous output signal.